Conventionally, it has been known that surface tension of fluid may vary every moment after a new surface is generated.
A typical example is a surface-active agent solution. It is a well-known phenomenon that, after its new surface is generated, molecules of the surface-active agent diffuse inside the fluid, reach the surface and adhere to the surface, thereby the surface energy, i.e., the surface tension of the solution gradually reduces. The surface-active agent solution is industrially a very important material. For example, surface-active agents are often blended with inks used for inkjet type printers, so that ink drops may immediately wet and permeate paper immediately after the ink drops being ejected and attached the paper.
Thus, for example, it is very important, in the inkjet technology, to know temporal variation of the surface tension of ink droplets which varies after ejection in order to design nozzles for ejecting ink or to know wettability of ink after attaching to paper.
In cases of the surface-active agents often used industrially, typical time necessary for the surface tension variation by the above-mentioned adhesion ranges between units of is (microsecond) and units of s (second), in their solutions of practical concentration. Specifically, in the solutions of sufficiently practical concentration, it ranges between 10 [μs] and 100 [ms].
On the other hand, there are some methods or devices which measure the time-varying surface tension (For example, refer to Patent Document 1.). Specifically, some methods, such as an oscillation jet method, a maximum bubble pressure method, and a surface tension wave measurement method, have been known.